A generator includes a shaft and a rotor body attached to the shaft and containing a plurality of poles. The number of poles is dictated by the speed at which the shaft will be turning and the frequency of the electric current to be produced.
Copper wire is wound on the poles and is referred to as the winding. The wire, in the case of large generators, is composed of flat, stiff, coiled copper bars, each roughly 1 inch by 0.25 inches in cross section, known as conductors.
The winding needs to form a complete circuit from the time it enters the first pole to the time it exits the last pole. Since the winding is made of such a stiff material in large generators, it is convenient to connect ends of the winding between adjacent poles with jumpers known as rotor pole crossovers.
Crossovers have taken many shapes and sizes as designs and needs have changed. For example, rings encircling the shaft, or shorter crossovers in the form of flat plates or reverse S-shapes oriented to lie axially relative to the shaft, have been used.
During operation of the generator, great centrifugal forces are exerted on, among other things, the winding and the crossovers. Further, the crossovers undergo stressful cyclic duty. That is, the generators may be started up and shut down on daily cycles to accommodate peak and off peak electrical generation demands, respectively.
Known crossovers have been found to suffer from a lack of flexibility. As a result, they crack under the centrifugal forces and cyclic duty encountered during use of the generator. Some crossovers actually crack all the way through. Cracked-through crossovers will cause a loss of generator electrical field.
Repairing a cracked crossover requires returning the generator to the factory, removing an end plate and retaining ring of the generator, replacing the cracked crossover with a new crossover, and re-attaching the retaining ring and end plate. Further, blocking used in the generator may have to be modified. These steps significantly increase cost and down time of the generator and perhaps the system with which it is used.
Further, electrical requirements necessitate insulation of the copper winding from the inward shaft and from the outward retaining ring of the generator. The insulation used in machines of 1950 vintage can be irreparably damaged by the removal of the end plate and retaining ring for crossover replacement. Damage to the insulation renders the generator useless and requires that the generator be disassembled entirely and rewound. Rewinding is very expensive and amounts to a lengthy in-factory process.
There is also known a laminated crossover having a U-shaped portion with two leg extensions. This crossover connects to the winding ends via respective lap joints at the free ends of the legs.
This crossover differs from the flat plates or reverse S-shape crossovers discussed above, in that the installed laminated crossover extends radially outward with respect to the shaft, as opposed to being oriented axially with respect to the shaft. This laminated crossover cannot mechanically be configured to be installed axially. That is, if the lap joints are connected to the winding ends, any attempt to rotate the laminated crossover to be oriented axially would exert a great load on the laminations and they would buckle.
With this laminated crossover, as with the other crossovers described above, cracking is still a problem and replacement thereof requires that the generator be returned to the factory, the retaining ring and end plate removed and blocking modified. As noted above, this increases maintenance costs, down time and potentially damages generator parts, particularly the insulation. As such, the laminated version appears applicable only to new generator manufacture or rewinds, not to on-site replacement.